U.S. Secretary of State John Kerry said Saturday that Syrian President Bashar al-Assad must step down, but that the timing of his departure has to be decided through negotiation.

Speaking in London after talks with British Foreign Secretary Philip Hammond, Kerry urged Russia and Iran to use their influence in Syria to persuade Assad to negotiate his eventual departure in order to end to the country's catastrophic civil war.

Kerry also stressed the link between the Syrian civil war and the vast migrant flow of Syrians into continental Europe. He called the four-year war the "root cause" of the migration crisis, while calling the mass migration the main "challenge to continental Europe."

"We need to get to the negotiation," Kerry said. "That's what we're looking for, and we hope Russia and Iran and any other countries with influence will help to bring about that. ... So we talked about a number of ideas ... for how to use this moment when Russia appears to be more committed to doing more" against Islamic State extremists battling for control of Syria.

"That's the objective," he said.

Moscow and the West have sharply disagreed on strategy and tactics for ending the civil war, which erupted in 2011. Western governments have insisted the crisis will not end until Assad cedes or is driven from power, while Moscow argues that the threat from extremists must be quelled before addressing Assad's future.

An estimated 250,000 Syrians have been killed as diplomacy stalled. Hundreds of thousands of others threatened by IS forces and government troops have fled, seeking the relative safety and prosperity of continental Europe.

Syrian government troops killed

Separately, a Syrian watchdog group said Islamist insurgents had killed 56 members of the Syrian government forces in the country's northwestern Idlib province. The Britain-based Syrian Observatory for Human Rights said Saturday that the government fighters were shot "execution-style" at the Abu Duhur military airport.

Abu Duhur military airport, Idlib province, Syria

The monitoring group said the killings happened earlier this week, but it took several days to confirm the deaths.

The air base had been one of the Syrian military's last remaining strongholds in the province when it was captured on September 9 by Islamist rebels, including the al-Qaida-linked al-Nusra Front.

The Islamist coalition, the "Army of Conquest," took over the city of Idlib, which is the provincial capital, and several other areas earlier this year.